fix errors on jsPsych reference documentation page

[becky-gilbert]

Need to fix whatever happened here:
https://www.jspsych.org/core_library/jspsych-core/#var-trial-jspsychcurrenttrial-consolelogthe-current-trial-is-using-the-trialtype-plugin
And addNodeToEndOfTimeline shouldn't have an example with a callback: https://www.jspsych.org/core_library/jspsych-core/#examples

[becky-gilbert]

https://www.jspsych.org/core_library/jspsych-core/#this-code-would-be-in-a-plugin-jspsychfinishtrialcorrect_response-true

https://www.jspsych.org/core_library/jspsych-core/#var-progress-jspsychprogress-alertyou-have-completed-approximately-progresspercent_complete-of-the-experiment

[becky-gilbert]

The callback example has now been removed for the addNodeToEndOfTimeline docs and the formatting has been fixed via 163f347. I think the problem was that code blocks must have a line break after the ending backticks (```).